The present invention relates to a method of determining a stratification of an electrolyte with varying acid density and/or sulfate fractions in the active mass of the positive electrode plates in rechargeable batteries.
The invention also relates to a charging device for rechargeable batteries with a temperature sensor for measuring the battery temperature, a voltmeter for measuring the terminal voltage of the rechargeable battery and with a charge controller for carrying out the aforementioned method.
There is a need for determining the state at any given time or predicting the state of an electrochemical storage battery, such as for example the state of charge or the heavy-current load capacity. For example, the capability of a starter battery to start a motor vehicle with an internal combustion engine is governed by the state of charge and the state of aging or by the drop in capacity that has occurred, since the current level which can be drawn from the starter battery and the power which can be discharged are limited. It is particularly important to determine the state of charge or the starting capability of a starter battery in cases in which, for example, the engine is operated intermittently, since the electrical power supply system of the vehicle, together with its loads, is then still operated in periods in which the engine is switched off, although the generator is not producing the electrical power. In such cases, the monitoring of the state of charge and the starting capability of the storage battery must ensure that the energy content of the storage battery always remains sufficient to still start the engine.
One problem is that the stratification of an electrolyte, with varying acid density, that frequently occurs in the case of lead-acid batteries, and sulfation of the active masses of the positive poles as a result of the stratification, greatly changes the electrical behavior of the battery. Depending on the degree of acid stratification, the amount of charge that can be taken from the battery may for example decrease by half. The acid stratification occurs substantially when the batteries are charged, in that the electrolyte is concentrated with sulfate ions directly at the plates and collects on the lower part of the battery cell due to the higher specific density in comparison with the surrounding electrolyte. This uneven distribution of the sulfate ions involved in the electrochemical reactions leads to a different behavior than with evenly distributed concentrations, and to a reduction of the service life.
DE 101 03 848 A1 describes a method of determining the aging of a lead-acid battery in which various operating parameters, such as terminal current, terminal voltage, temperature, relative current intensity and voltage per cell, as well as operating states such as degree of discharging, cycle amplitude, time in the discharged or charged state, acid density change and existing acid stratification are combined with one another with the aid of fuzzy logic algorithms. The reversible aging mechanism of the acid stratification is indicated as a function of the operating parameter of the relative current and the battery states of the degree of discharge and the cycle amplitude. In this case, the acid distribution occurring within a time interval along a battery cell is considered.
Furthermore, EP 1 505 402 A describes a method of predicting electrical properties of an electrochemical storage battery in which the state of charge is determined by two different methods based on a first and second parameter. Allowance is also made here for the value of the acid density, which has a different effect on the converted amount of charge than on the state of charge value based on the off-load voltage as the second parameter.
It is disadvantageous that it is always necessary to wait several off-load periods before performing an assessment of the stratification.
Accordingly, there is a need to provide an improved method of determining a stratification of an electrolyte with varying acid density and/or of sulfate fractions in the active mass of the positive plates and also an improved charging device for rechargeable batteries, in order to be able to detect stratification in a rechargeable battery within an extremely short time, without the need for lengthy observation of the battery, and distinguish a stratified rechargeable battery from an unstratified rechargeable battery.